It is often necessary, or at least desirable, to concentrate a liquid mixture by removing a portion of the solvent, generally water, from the liquid mixture. The resulting product, therefore, is in a more concentrated form. It has been common to concentrate fruit and vegetable juices such as orange juice, grapefruit juice, grape juice, and tomato juice by evaporation to remove water. In addition, seawater and brackish water have been concentrated by evaporation, although the condensed vapor has been recovered as usable potable water rather than discarded as in concentrating fruit and vegetable juices. Nevertheless, each is a concentrating process. In the case of juice, the concentrate is the desirable product whereas in obtaining potable water from seawater or brackish water the concentrate is discarded.
Evaporative concentration as described, as well as evaporation of chemical solutions or liquid dispersions, requires substantial energy since it relies on the latent heat of vaporization. Scaling of equipment and enhanced corrosion are often inherent at the temperatures involved in evaporative concentration. Loss of flavor and aroma also result during evaporative concentration of food products.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many products, particularly those having water as the liquid carrier. Generally, reduced energy is required since freeze concentrating relies on the heat of fusion instead of the heat of evaporation. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate in a concentrator or separator vessel. Next, the ice crystals are washed in a washer vessel to remove the concentrate remaining on them. The ice crystals can then be discarded or melted if potable water is desired.
Ogman U.S. Pat. No. 4,091,635 in part discloses freeze crystallizing in one vessel and then feeding an ice slurry to a wash column where the ice is separated and washed. This patent, however, does not teach a system of recycling the aqueous concentrate from the wash column to the freeze crystallizer vessel in which the ice is formed. Such recycling is believed to have been avoided as impractical with the previously available freeze crystallizers, perhaps because of ice bulid-up problems.
Recently, my associates have developed a freeze exchanger of the shell and tube structure in which an aqueous liquid mixture can be indirectly cooled, as it flows through the tubes, by heat exchange to a circulating cold fluid, usually a refrigerant, on the shell side. Such a freeze exchanger is diclosed in the copending U.S. patent applications of Engdahl Ser. No. 160,112 filed June 16, 1980 and Stafford et al. Ser. No. 191,357 filed Sept. 29, 1980 and Ser. No. 197,482 filed Oct. 16, 1980. Ice bulid-up in the tubes is avoided, or at least greatly reduced, by highly polishing the tube surfaces. Another way is to coat the tube surfaces with a substance to which ice has a very low adherence.